1. The Field of the Invention
The present invention relates to analog integrated circuit design, and more particularly, to up-converting a down-converted baseband signal in a direct conversion architecture without the baseband signal passing through active elements.
2. Background and Related Art
Electrical signals have proven to be an effective means of conveying data from one location to another. The further a signal is transmitted, however, the greater the decay in the signal and the greater the chance for irreversible loss in the data represented by the signal. In order to guard against this signal decay, the core electrical signal that represents the data (i.e., the baseband signal) may be modulated or superimposed on a carrier wave in the Radio Frequency (RF) frequency spectrum.
In order to properly interpret the signal, conventional RF receivers extracts the baseband signal from the received signal. The data represented by the extracted baseband signal may then be interpreted by other downstream circuitry. In order to perform this extraction, typical receivers include circuitry which first converts the received radio frequency modulated signal into an intermediate frequency (“IF”) signal. This intermediate frequency signal is then converted into the baseband signal for further data processing. Receiver architectures that convert through the intermediate frequency are often called “heterodyne” receiver architectures. Naturally, circuit elements (called “IF components”) are required in order to deal with the intermediate conversion to and from the intermediate frequency.
It is desirable to reduce the cost, size, and power consumption of a particular receiver architecture design for strategic marketing of the receiver. This is particularly true of wireless RF receivers since those receivers are often portable and run on battery power.
One technology developed in order to reduce RF receiver cost, size, and power consumption is called “direct conversion.” Direct conversion refers to the direct conversion of RF modulated signals into corresponding baseband signals without requiring conversion through the intermediate frequency. Such direct conversion receiver architectures are often also called “zero-IF,” “synchrodyne,” or “homodyne” receiver architectures.
FIG. 4 illustrates a conventional direct conversion circuit 400 in accordance with the prior art. The circuit 400 includes an antenna 401 which receives the RF modulated signal. The antenna 401 then provides the received signal to an amplifier 402 which amplifies the signal for further processing. The amplifier 402 may be, for example, an RF low noise amplifier.
The amplified signal is then split into two branches, an “in-phase” branch 410, and a “quadrature-phase” branch 420. Each branch includes a mixer that initially receives the amplified signal. For instance, the in-phase branch 410 includes an in-phase mixer 411, and the quadrature-phase branch 420 includes a quadrature-phase mixer 421. A local oscillator 430 provides a sine or square wave signal as a control signal to each of the mixers. Each mixer is configured to nonlinearly process the amplified signal and control signal, resulting in output signal components at frequencies equal to the sum and difference of amplified signal and control signal frequencies, plus higher-order components at other frequencies. The circuit includes a 90-degree phase shifter 431 which causes the control signal for the quadrature-phase mixer 421 to be 90 degrees out of phase with the control signal for the in-phase mixer 411.
The signal from the in-phase mixer 411 is then passed through a low pass filter 412 to a baseband amplifier 413 to complete the extraction of the baseband (difference frequency) signal from the received signal as far as the in-phase branch 410 is concerned. Likewise, the signal from the quadrature-phase mixer 421 is passed through a low pass filter 422 to a baseband amplifier 423 to complete the extraction of the baseband (difference frequency) signal as far as the quadrature-phase branch is concerned. The in-phase and quadrature-phase baseband signals are then processed by signal processing circuitry 450.
The direct conversion circuit of FIG. 4 does not convert through an intermediate frequency and thus there are no IF components needed to deal with an intermediate conversion. Consequently, the direct conversion circuit of FIG. 4 is smaller, and requires less power than conventional heterodyne receiver architectures. Furthermore, the direct conversion circuit does not have to deal with image suppression as much as do heterodyne receivers.
However, there are some performance issues for the direct conversion circuit of FIG. 4 that limit its practical implementation. In particular, conventional direct conversion circuits suffer from Direct Current (DC) offset and 1/f noise. The DC offset in conventional direct conversion receivers has two primary origins. One primary origin is the down-converting mixer itself in which the DC offset are self-mixing DC products present at the mixer outputs. The other primary DC offset origin is in active elements, such as highly sensitive amplifiers, that operate on the down-converted baseband signal. Such DC offsets can swamp out the much weaker desired signal.
The active element (such as the active mixer and the highly sensitive amplifier) that operates on the baseband signal may also introduce significant 1/f noise. 1/f noise refers to a phenomenon whereby the level of noise introduced by an active element is much higher when operating on signals having a frequency spectrum that favors lower frequencies. Baseband signals tend to have frequency spectrums that tend towards lower frequencies as compared to the modulated form of the signal. Accordingly, the active elements that operate on the baseband signal in direct conversion circuits tend to introduce high levels of noise thereby adversely degrading the signal-to-noise ratio.
Direct conversion circuits have many advantages in terms of size, power consumption, and cost to manufacture. In addition, direct conversion circuits also have good selectivity and the mitigation of an image frequency. However, as mentioned above, many direct conversion circuits also have some drawbacks in the form of increased DC offset and 1/f noise. According, what would be advantageous are direct conversion circuits that have reduced exposure to DC offset and 1/f noise.